How to use a PBR Material’s Mask Image
Masks are used to mix other things. Mask is usually a grayscale image that controls how much of one thing you can see through another. You can use masks to mix different textures, colors, shaders and you can even mask effects you apply to something with them so it's really hard to tell what this was originally supposed to be used for. You could try to see what feature it matches on the material and see if it might be useful to you for anything, but it may also be that it is included by mistake and is not needed for anything in this particular case.